Pneumatic and hydropneumatic springs are in common use. Springs in which the piston rod is biased towards an outermost position are used e.g. as balancing means for trunk lids of motor vehicles. On the other hand, there is also sometimes need for pneumatic and hydropneumatic "tension springs" in which the piston rod is biased by the pressurized gas towards an innermost position. Such gas springs are useful e.g. in cases in which an object is suspended and is to be adjusted to different levels.